The conventional three-way catalyst for the cleaning of the exhaust gas of internal combustion engines removes the exhaust components HC, CO and NOx in a very efficient manner, however, only under the predominant condition in the driving cycle that the exhaust gas composition approximates the stoichiometric air-fuel ratio. For oxidizing exhausts, such as in lean mix engines and diesel engines, the NOx adsorption/reduction catalyst has gained wide acceptance, the catalyst operating according to the following principle: during the predominant phase of lean exhaust output in the driving cycle, NOx is accumulated in the adsorber material to be then desorbed and reduced in a rich pulse. Such an NOx catalyst is described in European Patent Application No. 0 560 991. Alkali metals, alkaline earth metals, rare earth metals and noble metals are named therein as adsorber substances which are applied to a carrier material such as aluminum oxide. The adsorption of oxygen in the form of O2xe2x88x92 in the lean phase is named as an assumed reaction mechanism, the O2xe2x88x92 reacting with the NOx from the exhaust to form NO2. A portion of the NO2 formed is further oxidized and diffused in the adsorber material in the form of nitrate ions. In the event of a shift of the exhaust composition to rich, the above-named reaction sequence takes place in the opposite direction, i.e., NOx is reduced to free nitrogen with the reducing components HC and CO of the richer exhaust. The catalyst operates predominantly in the lean phase; the time ratio of the lean phase to the rich pulse is approximately 50:1.
A significant problem is the ability to identify when the storage capacity of the adsorber material is exhausted and the rich pulse must be initiated. For this purpose according to European Patent Application No. 0 560 991, the cumulative speed of the vehicle is measured and the degree of charging of the adsorber material is estimated from it. Exact knowledge of the degree of charging is not possible with this method.
The sensor according to the present invention has the advantage that the instantaneous degree of charging of the storage material of the catalyst is measured and a quasi-linear control for the storage of NOx is made possible.
The ability to precisely monitor the degree of charging of the storage material makes it possible to better utilize the storage capacity, and the catalyst can be dimensioned in a more cost-effective manner. The conversion phase of the rich side can be predicted more precisely and consequently be integrated more favorably into the driving cycle.
Having the storage material of the catalyst serve at the same time as the base element for the sensor for charge measurement results in a space saving and cost saving method in relation to a sensor located downstream of the catalyst.
In a favorable manner, the sensor is constructed in such a way that it responds to an electrical or electromagnetic characteristic of the storage material which changes with the adsorbed quantity of NOx. In a particularly simple manner, the sensor can be designed as a capacitor, the capacitance of which changes with the dielectric constant, which in turn is a function of the degree of charging of the storage material.
In an advantageous manner, the sensor may also be designed as a resonator, a cavity resonator or a filled waveguide; in contrast to a planar capacitor, this makes more voluminous structures possible which are capable of containing a greater quantity of NOx. In addition, the influence of the storage material on the electrical or electromagnetic value to be measured is intensified, since it is possible to measure at a higher frequency.
A capacitor is preferably suitable for lower frequencies; if the plate dimensions approach the order of magnitude of the wavelength of the electromagnetic waves, the signals can no longer be interpreted easily.